The present application relates to systems and methods for process control, and more particularly to control of processes related to the amount of fluid in a container subject to externally-excited motions, and most especially to control of processes such as storing and blending of fluids and solids when such processes are conducted aboard a waterborne apparatus.
The following paragraphs contain some discussion, which is illuminated by the innovations disclosed in this Application, and any discussion of actual or proposed or possible approaches in these paragraphs does not imply that those approaches are prior art.
Background: Process Control of Fluid Level
Modern hydrocarbon production and processing systems use automated equipment in many aspects of their operations. A key element of such automated operations is the process control system. Such control systems usually include sensors, which in one important function, send a signal back to a process control unit. The signal is usually a communication of a process variable or condition, such as temperature, pressure, fluid or solid flow rate, or fluid level in a container. The process control unit can perform such functions as monitoring the system or unit operations or process variables, making process decisions, energizing and de-energizing equipment, collecting and maintaining data, providing stability to the overall system, and, very importantly, ensuring the system and the unit operations comprising the system operate within desired parameters.
Sensors are often placed at specific locations within a production or processing system to provide information necessary for the control of various unit operations. The role of the sensor includes providing accurate and real-time information about a particular process variable or set of variables such that the process control unit can make decisions based on good and accurate information.
As an example of the needs and challenges in the accurate sensing and control of processes in the production of hydrocarbons, consider the process of “completing” an off-shore hydrocarbon well-bore. Such work is usually managed and done aboard “service ships,” including the cementing of the well-bore.
After drilling of an oil or gas well-bore is finished, the well-bore is “cemented” under carefully-controlled conditions. The first step is to run a steel casing into the well-bore, followed by pumping cement into the annulus between the casing and the cylindrical bore-wall comprised of earth, stone, sand, and/or other subterranean substances and formations. In normal primary well cementing, a hydraulic cement composition is pumped down through the casing, and then (through e.g. a cement shoe at the bottom of the string of casing) up into the annulus. The cement sets in the annulus, forming an annular sheath which not only physically supports and positions the pipe string in the well-bore, but also bonds the exterior surfaces of the pipe string to the walls of the well-bore, thereby blocking the undesirable migration of fluids along the well-bore. The successful cementing of a well-bore is affected by many factors, including cement setting time.
Cement setting time is affected by many factors, including weight percent of various components in the mixed cement, such as components which speed or slow the setting time. When mixing the cement prior to pumping the cement down-hole, control of the weight percent level of such time-of-set-regulating components in the final cement mix can be critical to the success of the cementing operation. Too much of, e.g. a cement-set accelerator, can result in setting the cement too quickly and not allowing it to completely fill the annulus. Therefore, the cement as-mixed must be provided within specific cement recipe control parameters. The cement is usually mixed in a blender which receives water, dry cement, dry powders, and/or special additives such as cement-set-time accelerants. Such mixing is often done continuously, rather than by batch. In a continuous mixing system, the blender receives the various material inputs and produces a mixed cement output. Sensors might monitor the flow rate of the water, flow rate of the dry and mixed cement, pressure, viscosity, density, and other measured process variables, and this information can be fed to the process control unit which monitors and then makes decisions to control the cement blending process system. For example, the process control unit might seek to maintain the fluid level in the blender at a constant level. Because the pumping of the cement into the annulus can speed-up, slow down, start, and momentarily stop from time to time, the process control unit will need to adjust the material inputs based on the output being sent down-hole to keep the blender level constant. If the level in the blender goes high as sensed and signaled by a fluid level sensor, the process control system can, for example, either momentarily stop or slow the addition of materials into the blender.
In a typical situation, the blender can be sized to hold only about fifteen seconds of mixed cement demand. Thus, if the process control system makes an under-shoot error, or is too slow in responding to a low level measurement signal from the fluid level gauge in the blender, the blender can possibly run empty, and the pump could suck and pump air into the annulus. In another situation in which a low fluid level is sensed, the process control system might over-shoot the addition of a particular component, such as a cement-set accelerant, and seriously jeopardize the success of the cementing operation, because the cement might set-up before the annulus is completely filled. Modern fluid level control systems for the processing industries have been developed to handle many such under-shoot and over-shoot situations. However, most, if not all, have been developed based on the assumption that, for example, the blending process is not moving, or subject to accelerations and decelerations, as will be encountered if the operation is conducted on a ship on a river, lake, sea, or ocean, where it is subject to wave and/or wind actions. So, for example, when a hydrocarbon production maritime service vessel is conducting the cement blending operation, a wave in the ocean will move the ship, and the blender will move with the ship since it is physically attached to the ship, and the fluid in the blender, or in most containers, for that matter, will “slosh”. A significant problem then arises, for example, in the cement blending operation, because the fluid level sensor in the blender can sense a low level when in-fact the blender has the correct amount of mixed cement. For example, the mixed cement might slosh such that the side of the blender where a fluid level sensor is located has a low level and the other side has a high level, at least momentarily. As sloshing continues, the level sensor unknowingly continues to send the changing level signal to the control system, which if not corrected, will begin to make under-shoot and over-shoot errors.
A conventional approach is to average the fluid level signal over some period of time. However, in the case of ocean swells, for example, the time between swells can easily be fifteen to sixty seconds. Thus, if an averaging process control approach is used based on a sixty second running average to “damp” the effect of the ocean swells, the process control system would be tuned so “slowly” as to miss any “true” or actual fluid level deviations caused by process upsets or changes, since, for example, the blender may hold only fifteen seconds of mixed cement. In this instance, “true” or “actual” is used to mean that the level deviation is due to the process and not due to sloshing caused by accelerations of the container. Thus, averaging, damping, or lagging the control system to account for slow-moving ocean swells is not a completely adequate solution to the problem.
Note that a maritime ship, vessel, or apparatus can move in six ways. In this instance, “maritime” refers to waterborne operations conducted on either fresh or salt-water. See FIG. 1A for a depiction of each movement of, for example, a ship 100 with a partially filled container of fluid 100A on it. The ship, and thus the container, can surge (e.g. 104 front to back shifting), heave (e.g. 101 up and down “bobbing”), sway (e.g. 106 side to side shifting), pitch (e.g. 103 front to back rocking), roll (e.g. 102 side to side rolling), and yaw (e.g. 109 rotating about a vertical axis). Each movement can also have a different amplitude, velocity, period, and acceleration or deceleration. For example, rolling of ships up to 30 degrees can occur, and accelerations can approach 0.5 to 1 g. Note that, of course, the motions can be random, variable, or periodic. Because these motions will also affect, for example, the sensed level of fluid in, for example container 100A, the motions can “confuse” a conventional fluid level control system.
Note that other fluid processing operations utilizing fluid level control are also commonly carried-out on-board maritime ships, vessels, or apparatuses, including subterranean reservoir fracturing fluid preparation and well drilling mud operations, as well as simply holding such fluids in tanks, vessels, accumulators, silos, or other such containers. Note also that such ships can vary in size and operate in waters of various degrees of movement. These factors also present the control system with differing and complex control problems.
Control systems to account for the motion of physical objects such as tall buildings in the wind or in an earthquake can use a combination of motion sensors and heavy movable weights. For example, as a building begins to sway in one direction, the motion sensor senses the sway and a process control unit sends a signal to a device to move a heavy weight in an opposite direction in an attempt to counter-act the wind or quake force, and thereby reduce the sway. This technique is commonly referred to as “active motion compensation”. One might envision that such a system could be applied to a maritime vessel to stop the six degrees of motion of the vessel itself. However, the size and complexity of the weights or compensating mechanical devices to correct for not just sway, but for the other five motions, represent significant engineering and other kinds of challenges.
One might also envision active motion compensation practiced on the fluid container itself, where the container 100A would be placed on a device or equipped with devices to practice active motion compensation. Again, the size and weight of such containers present engineering and other challenges to account for all six motions. Additionally, the pipes, feeders, pumps, mixers, and other equipment attached to the container or blenders would need to somehow be flexibly connected to the container being moved in response to active motion compensation, which will present additional engineering and other challenges.
Additional information related to compensating for noise in fluid height measurements in containers can be found in U.S. patent application Ser. No. 11/029,072, entitled “Methods and Systems for Estimating a Nominal Height or Quantity of a Fluid in a Mixing Tank While Reducing Noise,” filed on Jan. 4, 2005, which is incorporated by reference herein in its entirety.
Thus, solving the problem of reliably controlling processes involving fluid level control in containers subjected to externally-excited motions presents challenges, and requires solutions not adequately met by current approaches, and it remains a particular problem in off-shore equipment operations. Finally, there is an increasing need for reduction of uncertainty in the production of petroleum as the value of petroleum continues to rise.